Gloss is one quality evaluation item of an image sample produced by various techniques. In general, the gloss depends largely upon geometry (that is a positional relationship of a light source, a sample (hereinafter, the image sample is denoted simply as sample) and a light receiver) under which observation is carried out. The larger a zenith angle 1 in a light source incidence direction (in which the light comes from the light source) and a zenith angle 2 in a light reflection direction (in which the light goes to the light receiver) as illustrated in FIG. 2, the stronger the gloss that a person feels. In order to evaluate this gloss quantitatively as glossiness, presently a glossimeter adopts some limited kinds of arrangements (JIS Z 8741) such as a combination of a zenith angle 45° in the direction, in which the light enters toward a sample 3 and a zenith angle 60° in the light reflection direction. This arrangement is standardized in, for example JIS (Japanese Industrial Standards), and the like.
However, this technique gives glossiness that can be merely a kind of standardized measure, but can not provide quantitative data enough for evaluating a property of deviation reflection. In order to solve this problem, a gonio-spectro photometer system (gonio-photo spectrometer) and the like, which is generally used in painting industry, is used. This makes it possible to obtain quantitative data of the property of deviation reflection. However, the quantitative measurement using this gonio-spectro photometer system of an angle of deviation takes very long to obtain the measurement and can handle only limited varieties of shapes of a sample. Accordingly, this measurement is not so suitable for practical use.
In recent years, in a field of remote sensing, BRDF (Bidirectional Reflectance Distribution Function) is draws attention. This is devised based on Shafter's Dichromatic Reflection Model (refer to Document 1). In the Dichromatic Reflection Model, as illustrated in FIG. 3, reflection light from a surface of an object is made of two components called (i) a surface reflection (light component reflected on a surface) 4 and (ii) an internal reflection (light component reflected inside) 5. The surface reflection 4 is a light beam reflected on a surface of the sample 3 due to a difference in refractive indexes of the sample 3 and the air and has color of a light source 6. The light that enters inside the sample 3 is repeatedly refracted, absorbed, and scattered among dye particles 3A, whereby the light is absorbed into the dye particles 3A depending on the wavelength. Accordingly, the internal reflection light 5, which is a reflection from the sample 3, has a color of the sample 3. Various proposed models of the BRDF are used according to respective purposes.
Document 2 discloses a method for evaluating a property of deviation inside reflection. This method simulates an amount of specular reflection light, by using BRDF. With this method, it is also possible to simulate glossiness in geometry other than existing geometry.
However, in the above method, the amount of the specular reflection light received by a glossimeter is examined only by the surface reflection in the Dichromatic Reflection Model and the specular glossiness is calculated by the BRDF. In an image made of a concentrated colorant material and thus producing high gloss, it is possible to ignore the internal reflection light component in the Dichromatic Reflection Model and a reflection light component from a base material positioned under a colorant material layer. However, in a case of an image whose color density from a colorant material is low, the reflection light component of an lower layer cannot be ignored. Moreover, in case of a low gloss image, the internal reflection light component cannot be ignored. Therefore, a correct value cannot be calculated with the above method.
(Document 1)
COLOR Research and application, Vol. 10, No. 4, pp. 210-218, 1985
(Document 2)
Japanese Unexamined Patent Publication 2003-329586 (published on Nov. 19, 2003)